Proceeding Articles

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We report the cationic polyelectrolyte (CPAM)-SiO2 nanoparticle (NP) interactions in suspension and in a sheet form, when mixed with microfibrillated (MFC), using dynamic light scattering (DLS) and small angle X-ray scattering (SAXS) techniques. The CPAM-SiO2 NP suspensions were prepared by adding NPs into CPAM drop wise and composites were prepared by adding CPAM-SiO2 suspension into MFC and through standard paper making procedure. DLS revealed that increase in CPAM dosage creates larger sized CPAM-NP aggregates because more NPs can be picked up by stretched CPAM chains. SAXS study revealed that CPAM-SiO2 NP assembly in the formed nanopaper fits well with a spherical core shell model (with SiO2 partially covered with CPAM) and sphere model (SiO2 alone) combined together. Understanding the interaction between polyelectrolyte-NP system through such scattering techniques enables us to engineer novel cellulose based composites for specific applications.

In this paper we give a literature overview on three different aspects of pulp fiber-fiber bonding. First we are reviewing how the adhesion between the pulp fibers is created by the capillary pressure during drying of a sheet. Second we are discussing the individual mechanisms relevant for fiber-fiber bonding. They can be grouped in three different groups: (a) The area in molecular contact, which also includes interdiffusion; (b) the intermolecular bonding mechanisms hydrogen bonding, Van der waals forces and coulomb interaction; (c) the mechanical bonding mechanisms which are capillary bridges and mechanical interlocking. The third and last part of the review discusses the failure process of fiber-fiber bonds and related single fiber-fiber bond testing methods. The general emphasis of the paper is set on providing a general understanding of the processes responsible for how bonds between fibers are created, how they work and how they are failing.

Wet paper that has been crumpled into a ball shows little tendency to recover to a planar shape when the applied pressure is released – a characteristic called poor wet resiliency. We report the results of an investigation into approaches to improve paper wet resiliency through the choice of fiber types and fiber treatments. Following the lead of the textile industry and the patent literature, wet recovery angle (WRA)  was used as a measure of wet resiliency. In this technique, wet strips of paper are folded, without creasing, pressed, and then released. The WRA was measured after the paper relaxed – the greater the WRA, the more resilient the paper.

All of our sheets with a WRA > 0° contained PAE, a standard wet strength resin. Except for sheets based on Abaca fibers, standard PAE or PAE + CMC (carboxymethyl cellulose) applications gave very low WRA. Instead, conventional fibers had to undergo wither TEMPO oxidation or CMC grafting before PAE application. Both fiber treatments substantially increase fiber surface charge density, promoting PAE adsorption and, more importantly, giving covalent bonding sites on the fiber surfaces for grafting PAE.

When comparing the WRA values from treated fibers (TEMPO oxidation or CMC grafting), the ranking was Abaca >>Lyocell > bleached southern softwood kraft > bleached eucalyptus kraft. For pulp mixtures, treated fiber contents of 20-40% had a much bigger influence on WRA compared to wet tensile strength.

Wet-resiliency is due to the swelling of the “hinge region” in folded wet paper, in combination with a sufficiently strong fiber network that can translate the swelling forces into shape recovery. Limited data suggests that the extent of recovery from z-directional wet compression is directly correlated with WRA values. By contrast, wet tensile strength is weakly correlated with WRA.

A novel methodology is developed to visualize and quantify biomolecules adsorption at the cellulose film0liquid interface. Hydrogenated cellulose (HC) films were made from cellulose acetate and deuterated cellulose (DC) films produced using deuterated bacterial cellulose. Deuterated bacterial cellulose was obtained by growing the Gluconacetobacter xylinus strain ATCC 53524 in D2O media. Horse Radish Peroxidase (HRP), a robust and well known enzyme, was selected as model functional biomacromolecule to adsorb at the cellulose interface. The film thickness and quantification of adsorbed HRP molecules were characterized by X-ray and neutron reflectivity (NR) measurements. Reflectivity data analysis reveals the cellulose films to be smooth (low roughness) and uniform. The HC and DC films are 206 A and 92 A thick, respectively, and both films swell in the aqueous buffer solution. In NR measurements, it is difficult to trace the adsorbed HRP layer on HC film due to the small scattering length density (SLD) difference between HC and HRP providing no contrast. However, using deuterated cellulose (DC) film provides sufficient SLD difference (contrast) with respect to the SLD of HRP. The adsorbed HRP layer is 110A thick and occupies a volume fraction of 20%. Using deuterated cellulose films enabled the quantification of thin and partial layers of proteins at the liquid interface. Quantifying and controlling the morphology and functionality of biomolecules at the cellulose interface enables to efficiently develop and optimize low cost cellulose based diagnostics devices with superior functionalization.

Reactivity is an important quality parameter form some grades of pulp, but its estimation is usually complex and time demanding. The study verified the possibility to utilize an alternative rheological approach to investigate pulp reactivity rapidly and with limited effort. 12.5 mL of pulp at 1.5% consistency was dissolved in 12.5 mL bis(ethylenediamine) pulp copper (II) hydroxide solution (CED) 1M The reaction was monitored by a torque rheogram and its two indices: the optimal dissolution time (ODT) and the initial dissolution rate (IDR). The method used to assess pulps either hornified to different extents or subjected to enzymatic hydrolysis. ODT was shown to decrease with increasing pulp swelling, while IDR reported the opposite trend. Both the indices were shown to be sensitive to pulp treatments. However, the ODT showed larger differences between hardwood and softwood pulps. The method had reasonable statistical reliability and required only 1.5 h per measurement.

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