1989 Volume 1
A concept for the fibre flocculation process in turbulent flow is presented including both rupture and aggregation of fibre flocs. Based on results from the literature, a mechanism is proposed for the breaking-up and the building-up of flocs in interaction with turbulent eddies of different length scales in the turbulence spectrum. The mechanism has been formalized into a two-way hierachical concept with analogy to the decay of a scalar fluctuation.
Deformation and break-up of flocs by turbulent eddies with a length scale similar to the floc scale dominates the rupture process. Eddies of a smaller scale agitate the floc causing weakening and activation of the network. Floc aggregation happens due to floc collisions when the flocs are transported by turbulent eddies of a larger scale. The smaller scales activate the floc surface, and increase the probability of forming a floc. Erosion and deposition of single fibres takes place in the turbulent fine structure.
This concept finds support when tested and compared with the results in the literature. Experiments have been conducted in a vertical pipe flow with a varying flow rate, fibre length and consistency, taking measurements at various positions downstream of an orifice . Results were obtained using high speed movie film, image analysis of still photos and measurements of flocculation and turbulence spectra using a laser . Increasing the turbulent kinetic energy or decreasing the macro scale gives lower floc intensity at large length scales .
During web formation, the difference between the speed of the pulp suspension and that of the wire gives rise to a smear force that orients fibres preferentially along the machine direction . This has been known for a long time. It is less clear, however, haw the distribution of fibre orientations is fly related to the conditions of web formation and to the properties of papermaking fibres. We have studied this question using simple theoretical models and experimental data inc1-tiding irk-We analysis results.
We find that fibre-to-fibre interactions determinehow much individual fibres are rotated by the oriented shear field. In this way the fibre orientation distribution obtained for a given speed cliff an the pulp properties. On the other hand, the turbulence of the pulp flow seems to be the mechanism through. which the conditions of web formation affect fibre orientation. In particular, we suggest that the on set of hear-induced turbulence in the suspension flow determine the maximum fibre orientation anisotropy that can be achieved.
The model calculations show that the distribution of fibre orientations cannot be represented in terns of a simple functional form with only one adjustable parameter. The shape of the distribution depends an the fibre p and especially the wet fibre flexibility. In paper the orientation distribution of curly fibres is quite different from that of straight fibres. On the other hand, no diff in the distribution is observed if long and short fibre are examined separately.
The fundamental physico-chemical aspects of retention chemistry are reviewed in the light of basic concepts in colloid chemistry.
Special emphasis has been paid to the surface chemistry of cellulose and cellulosic materials, their origin of charge, dispersion force interactions as well as the implication of certain aspects of peculiar cellulosic surfaces, e.g. the influence of their porosity on polymer adsorption.
Charge neutralization, patch flocculation, heterocoagulation, bridging and complex flocculation phenomena are discussed as well as polymer adsorption phenomena at the cellulose/water interface
Cambridgepp 413-435Conformation of Adsorbed Polymers and Flocculation of Microcrystalline Cellulose and Pulp SuspensionsAbstractPDF
This paper consists of three parts. In the first part, the reconformation of polyelectrolytes adsorbed on fiber surfaces has been studied by measuring simultaneously the amount of adsorbed polymer and the number of released counterions. It was found that the time for reconformation from an initial, extended conformation to a more flat conformation was of the order of 60 seconds for a high molecular mass polyacrylamide. In the second and third parts, the flocculation of suspensions of microcrystalline cellulose and pulp fibers has been investigated. The results of these flocculation experiments can be related to the conformation and adsorbed amount of the polyelectrolyte.
Cambridgepp 437-452The Use of Potentiometric Titration and Polyelectrolyte Titration to Measure the Surface Charge of Cellulose FibreAbstractPDF
The surface charge of cellulosic fibre was determined using two methods . The relative surface charge (PTC) of three bleached pulps was quantified using polyelectrol to titration and compared with the absolute surface charge (So) measured by potentiometric titration. It was established that the polyelectrolyte titration method was a less tedious, but valid method of determining the surface charge of fibre, providing the titration method is standardised and the effect of variables such as suspension, pH, fibre concentration, ionic content and polyelectrolyte molar mass are eliminated. The method was used to construct a charge/pH isotherm of a bleached hardwood fibre similar to those constructed using potentiometric titration. Furthermore, the effect of refining of bleached hardwood and softwood krafts on the PTC was investigated.
Cambridgepp 453-470Exchange of Cationic Polymers Adsorbed on Cellulose Fibers and on Monodisperse Polystyrene LatexAbstractPDF
Two series of cationic polyacrylamides (C-PAM) were prepared, with and without fluorescent labelling, one with constant charge density (CD:1.4 meq/g) and different molecular weights (MW:2×104, 4×105 and 8×106) and one with constant molecular weight (4×105) and different charge densities (CD:0.65 meq/g, 1.4 meq/g and 2.5 meq/g) .
The exchange reactions of these C-PAMs on monodisperse polystyrene latex (PSL) and cellulose fibers have been studied using C-PAMs with the same MW and CD except that the pre adsorbed polymers were fluorescently labelled. The first series of experiments was carried out near the saturation level of the pre-adsorbed polymers. For C-PAMs with the highest MW(8×106) no significant exchange could be detected on PSL or on fibers. When C-PAM with the highest. CD (2.5 meq/g) was used a slight exchange occurred on both PSL and fibers . The exchange became more extensive with decreasing MW and CD. The exchange reactions initially proceeded quite rapidly and then slowed down. The initial exchange was more extensive on cellulose fibers than on PSL, probably because of the lower surface CD of the fibers, but it levelled off more rapidly than on PSL. This difference is most probably an effect of the porous structure of the fibers.
Cambridgepp 471-494Physiocochemical and Hydrodynamic Aspects of Fines and Fillers RetentionAbstractPDF
From the results of model studies on well-characterized systems, valuable conclusions can be drawn regarding several phenomena occurring in papermaking suspensions relevant to fines and fillers retention. We have shown that long range hydrodynamic interactions are operating between small particles (such as fillers) and spheroids (such as fibers) subjected to simple shear, preventing small particles from approaching large ones to within distances where colloidal forces become important. We can expect similar effects in papermaking suspensions, resulting in very low efficiencies for the deposition of fillers or fines on fibers. The efficiency can be improved by high molecular weight polymers which adsorb on the particles and can reduce the minimum distance of approach between a filler and a fiber. From model experiments on the deposition of T102 particles on cellophane, it can be concluded that the electrostatic forces also play an important role in fines and fillers retention. Usually no deposition occurs far below a critical deposition concentration (CDC) of electrolyte, slow deposition occurs just before the CDC and fast deposition above the CDC. Charged polymers, such as cationic polyelectrolytes, are most effective in retention because they can reduce the gap between a filler and a fiber during an encounter, and they ensure that electrostatic repulsion is negligible. Resides increasing the efficiency of deposition, retention aids can also increase the bond strength between a filler and a fiber, thus preventing or minimizing the rupture of fiber-filler bonds.
The apparent light-scattering coefficient of a given pigment in filled papers depends on the state of pigment dispersion and the beating level of fibers. This effect is seen on handsheets containing pigments deliberately introduced either as individual particles or as aggregates; well dispersed T102 is optically superior and its contribution to the total light scattering increases with fiber beating. In order to achieve retention of dispersed pigment particles and to avoid their flocculation caused by polymeric retention aid, the pigment, prior to its addition to the fiber suspension, can be treated with a cationic polyelectrolyte. Since pigment introduction results in tensile strength loss, which is most pronounced with dispersed particles, a plot of optical properties versus tensile strength provides a means for evaluating the pigment effectiveness and for comparing different pigments.