Lars Ödberg

Colloidal Stability of Aqueous Nanofibrillated Cellulose Dispersions

Cellulose nanofibrils constitute an attractive raw material for carbon-neutral, biodegradable, nanostructured materials. Aqueous suspensions of these nanofibrils are stabilized by electrostatic repulsion arising from deprotonated carboxyl groups at the fibril surface. In the present work, a new model is developed for predicting colloidal stability by considering deprotonation and electrostatic screening. This model predicts the fibril-fibril interaction potential at a given pH in a given ionic strength environment. Experiments support the model predictions that aggregation is induced by decreasing the pH, thus reducing the surface charge, or by increasing the salt concentration. It is shown that the primary mechanism for aggregation upon the addition of salt is the surface charge reduction through specific interactions of counterions with the deprotonated carboxyl groups, and the screening effect of the salt is of secondary importance.

On the charge stoichiometry upon adsorption of a cationic polyelectrolyte on cellulosic materials

Abstract Adsorption isotherms and adsorption Stoichiometry of a low-molecular-weight (Mw = 5.9·103) cationic polyelectrolyte, 3.6-ionene (Polybrene), on cellulosic fibers were studied. The charge density of the fibers was varied by means of carboxymethylation. In order to study the effects of increased accessibility to the charges within the fiber cell wall, part of the fiber material was treated in a high-pressure homogenizer to produce a microfibrillated cellulose ( MFC ). It is shown that the adsorption isotherms are of the high-affinity type with a pronounced plateau level. The charge of the polymer adsorbed at this plateau level provide a relatively good measure of the charge (in meq g−1) on both the fibers and the MFC, except for the unsubstituted fiber sample. By measuring simultaneously the adsorption of polymer and the amount of released counterions it was also shown that there is an almost 1:1 Stoichiometry between the charges on the polyelectrolyte and the charges on the fibers. However, the Stoichiometry drops rapidly when the amount of adsorbed polymer charges exceed the number of charges on the cellulosic fibers. Finally, the deviation from a 1:1 relation between the total charge on the cellulosic material and the adsorbed polymer charges for the unsubstituted fibers is also shown to be caused by a slow adsorption process.

Polyelectrolytes adsorbed on the surface of cellulosic materials

Abstract The adsorption of two cationic polyelectrolytes onto carboxymethylated fibers of cellulosic pulp and rayon has been studied both by equilibrium measurements and in dynamic experiments with consecutive addition of the polyelectrolyte. For a low-molecular-mass polyelectrolyte, 3,6-ionene, the adsorption is rapid and the same result is obtained both in equilibrium and dynamic experiments except at low degrees of substitution. It is shown that there is almost a 1:1 stoichiometry between charges on the adsorbed polyelectrolyte and the total amount of carboxyl groups on the fibers which indicates that a cation exchange reaction is taking place. For a polyelectrolyte of a higher molecular mass, polydimethyldiallylammonium chloride (DMDAAC), the adsorption is lower and the resulting equilibrium adsorption is not stoichiometrically proportional to the charge on the fibers.

Adsorption of cationic polyacrylamides onto monodisperse polystyrene latices and cellulose fiber: Effect of molecular weight and charge density of cationic polyacrylamides

Abstract Series of cationic polyacrylamides (C-PAM) in which (a) the molecular weight (MW) was constant (4 × 10 5 ) and the charge densities (CD) varied (0.65, 1.4, and 2.5 meq/g) and (b) the CD was constant (1.4 meq/g) and MWs varied (2 × 10 4 , 4 × 10 5 , 8 × 10 6 ) were prepared with and without a fluorescent label. The adsorption of these C-PAMs onto monodisperse polystyrene latices (PSL) was very rapid regardless of MW and CD, but the adsorption rates onto cellulosic fibers were significantly influenced by MW and CD due to the porous nature of the fibers. A considerable degree of adsorption was, however, also rapidly achieved in the case of fibers. The ratio of the charge of the adsorbed polymer to the charge of the substrate is discussed on the basis of the adsorption isotherms obtained. The fluorescent labeling had little effect on the adsorptivity of C-PAM on either PSL or fibers, probably because of the low degree of substitution (

Kinetics of adsorpton and ion-exchange rections during adsorption of cationic polyelectrolytes onto cellulosic fibers

Abstract The kinetics and stoichiometry of adsorption opf a cationic polyacrylamide (C-PAM) and a 3,6-ionene on a series of carboxymethylated bleached cellulosic pulps here been investigated. The initial rate of adsorption of C-PAM was very fast regardless of the degree of substitution (DS) of the fibers. Approximately 50% of the equilibrium adsorption is reached already within 10 s after polymer addition. The degree of ion exchange between the cationic polymers and the anionically charged groups on the fiber was initially small and increased substantially when the equilibrium polymer adsorption was approached. With increasing DS and an increasing amount of added polymer a lower stoichiometry was reached after 30 min. Experiments with the 3,6-ionene show that this more highly charged polymer was initially adsorbed with a higher ion-exchange stoichiometry, and no large increase in stoichiometry can be detected with increasing time after polymer adsorption. These results indicate that the C-PAM polymers are initially attached by only a few segments to the cellulose surface and that a flatter conformation is obtained the longer the time of adsorption. The results from experiments with the 3,6-ionene indicate a flatter conformation of this polymer on the surface.

Effects of drying and pressing on the pore structure in the cellulose fibre wall studied by 1H and 2H NMR relaxation

A 1H and 2H NMR relaxation method was used to investigate the influence of drying and pressing on the pore size and pore size distribution in the cellulose fibre wall. The investigation was made in the moisture interval in which cellulose fibres normally shrink, i.e. from a moisture ratio of about 1.5 g water/g fibre to dry fibres. When the moisture content of a fibre sample was decreased by drying or pressing, the pores decreased in size and the pore size distribution became narrower. It was found that there were only small differences at a given moisture content between the pore size distributions of samples prepared by drying and by pressing. The results also indicate that the pore shrinkage in cellulose fibres during pressing or drying is a process in which the cell wall pores of a wet cellulose fibre successively shrink as the moisture content decreases. It was observed that, at low moisture contents, pressing and drying resulted in different 1H NMR spin-lattice relaxation profiles. This is discussed in terms of morphology differences in the fibre matrix. The mobility of the protons in the solid phase influences the liquid 1H NMR spin-lattice relaxation in heterogeneous systems through magnetization transfer. We have also studied the effects of hornification in recycled pulps

Spectroscopic Ellipsometry Characterisation and Estimation of the Hamaker Constant of Cellulose

Calculations of Hamaker constants using Lifshitz theory require the availability of accurate dielectric data, especially in the visible-ultraviolet region. We present spectroscopic ellipsometry data on well-defined cellulose films of a limited thickness range (100–140 layers) deposited on an oxidised and hydrophobised silicon substrate. The spectral data, representing measurements from a perpendicular orientation to the fibre deposition direction, was used for estimates of the necessary spectral parameters, i.e. the oscillator strengths and characteristic frequencies in the UV-range. Our calculations show that cellulose has a relatively low Hamaker constant in air (58 zJ) and water (8.0 zJ). The implications for the surface energy estimates of cellulose and colloidal interactions between cellulose and various types of fillers and coating colours are indicated.

Water diffusion in wood pulp cellulose fibers studied by means of the pulsed gradient spin-echo method

Abstract The self-diffusion of water sorbed in wood pulp cellulose fibers was studied by using the pulsed gradient spin-echo (PGSE) method. The observed echo attenuation profiles deviate significantly from those of bulk liquids and can be decomposed into two components: one with a self-diffusion coefficient independent of the diffusion time, and one with an apparent diffusion coefficient that depends on the diffusion time. The relative amplitude of these two components for the different diffusion times used in the PGSE experiment is nearly constant. We attribute the two components to bulk water between cellulose fibers and to water in pores within the fibers, respectively. From the relative amplitude of the components and the known water content in the samples, the amount of water in pores inside the fibers was calculated to 1.4 g/g of cellulose fibers. The self-diffusion coefficient of the bulk water between the fibers is lower than that of neat water. This reduction is mainly caused by the obstruction effect of the cellulose fibers whereas the hydration effect is of minor importance. The apparent self-diffusion coefficient of water trapped in pores inside fibers is approximately one-third of that of the bulk water between fibers when the diffusion time is 12 ms and is reduced further by another factor of 3 when the diffusion time is increased to 80 ms. By using a sheet sample and applying the magnetic field gradient perpendicular or parallel to the sheet it was found that the diffusional motion of water in pores is anisotropic. These results indicate that the pores are elongated along the fiber axis having lengths ranging from a few micrometers up to 20 μm.

An extended model for the estimation of flocculation efficiency factors in multicomponent flocculant systems

Abstract A model for the flocculation efficiency in flocculant systems consisting of up to three components (a low molecular weight (MW) cationic polymer, a high MW cationic polymer and an anionic particle sol) is presented. The model is based on the classical bridging mechanism. Extensions have been carried out to take into account the addition of low MW polymers making surface sites inaccessible for bridging polymers, the flocculation induced by the addition of anionic microparticles, and the conformation of the adsorbed high MW polymer layer. According to this model, the collision efficiency is a function of the degree of surface coverage of the suspended particles by low MW polymer, the degree of surface coverage by high MW polymer, and the addition of a third component which interacts with adsorbed high MW cationic polymers. In the experimental work, the model was tested by determining adsorption isotherms, by flocculation experiments on suspensions of cellulosic fibres and by experiments on a pilot paper machine. The model was found to give a good qualitative description of the experimental results.

Turbulent pipe flow studied by time-averaged NMR imaging : measurements of velocity profile and turbulent intensity

A time-averaged method to obtain quantitative measurements in turbulent flow by phase flow encoding NMR imaging is introduced. With this method time-averaged velocity profiles and turbulence intensities can be determined. Time-averaged velocity profiles for pipe flow of water driven by a constant pressure gradient at Reynolds numbers from 1200 to 9400 were visualized. A precise correlation between the pixel intensity of the time-averaged NMR flow image and the local turbulence intensity of the flow is derived. The measured turbulence intensities are in agreement with published data obtained using other experimental methods.