Tuomo Hjelt

Influence of colloidal interactions on pigment coating layer structure formation.

The consolidation of pigment coating layers was simulated using a three-dimensional particle dynamics model. The model included hydrodynamic interactions as well as colloidal force models and the Brownian motion. The impact of various colloidal model parameters on the z-direction solids profile development and coating layer thickness was investigated. Also, the influence of continuous (liquid) phase viscosity was tested. Particle systems resembling a polydisperse ground calcium carbonate (GCC) distribution were studied. Results show that a lower particle surface potential on pigments increased the thickness of the coating layer, while the electrostatic double layer thickness influenced the internal coating structure. An increased viscosity of the continuous phase slowed the consolidation down, but did not have a significant impact on the final microstructure. The work contributes to the understanding of the influence of colloidal system properties on the consolidation and structure development of coating layers. The results may aid in the understanding of the impact of chemical additives on coating layer structure formation.

A particle motion model for the study of consolidation phenomena

Abstract A three-dimensional method is described for simulating the dynamics of low particle Reynolds number colloidal suspensions at high solids content. The method is based on modified Stokesian dynamics and includes hydrodynamic interactions, colloidal interactions, steric interparticle forces from polymers and the Brownian motion. A free surface model, containing hydrodynamic and surface tension components, is also described. The applicability of the method ranges over a wide variety of particle systems, but in this work is tailored for the simulation of paper coatings. The particle suspension is represented by spherical rigid bodies of a given size distribution that are suspended in a Newtonian liquid. Numerical simulations performed using this technique enable the study of microscopic scale mechanisms, the characterisation of particle system constituents, an appreciation on microstructure development in time and the evaluation of macroscopic level properties of particle suspensions or consolidating systems. The method is evaluated in a number of test cases to illustrate its functionality and provide examples on the potential of its use in the simulation of paper coatings.

Fractionation of Nanocellulose by Foam Filter

The foam fractionation method was applied for nanocellulose. Experiments were carried out with enzymatically pretreated nano-fibrillated cellulose (NFC) from softwood, as well as commercial products. Narrow channels (plateaus) between bubbles prevent the flow of coarse particles along the water, so that foam acts like a filter. The advantage of the method is no risk of clogging, which could be a big problem for conventional filters or screens. Mean particle size (effective size by means of dynamic light scattering measurement) was reduced by foam fractionation, and the reduction range depended on the cellulose grade and the type of surfactant. The yield turned out to be low, probably because of particle aggregation due to the interaction with surfactant.

Porous structure of fibre networks formed by a foaming process: a comparative study of different characterization techniques

Recent developments in making fibre materials using the foam‐forming technology have raised a need to characterize the porous structure at low material density. In order to find an effective choice among all structure‐characterization methods, both two‐dimensional and three‐dimensional techniques were used to explore the porous structure of foam‐formed samples made with two different types of cellulose fibre. These techniques included X‐ray microtomography, scanning electron microscopy, light microscopy, direct surface imaging using a CCD camera and mercury intrusion porosimetry. The mean pore radius for a varying type of fibre and for varying foam properties was described similarly by all imaging methods. X‐ray microtomography provided the most extensive information about the sheet structure, and showed more pronounced effects of varying foam properties than the two‐dimensional imaging techniques. The two‐dimensional methods slightly underestimated the mean pore size of samples containing stiff CTMP fibres with void radii exceeding 100 μm, and overestimated the pore size for the samples containing flexible kraft fibres with all void radii below 100 μm. The direct rapid surface imaging with a CCD camera showed surprisingly strong agreement with the other imaging techniques. Mercury intrusion porosimetry was able to characterize pore sizes also in the submicron region and led to an increased relative volume of the pores in the range of the mean bubble size of the foam. This may be related to the penetration channels created by the foam‐fibre interaction.