Hetero-flocculation of wood fibres and fines induced by polymers and microparticles

The retention of process colloids in the paper web is essential for papermaking process efficiency and product quality. Fines (fibre fragments) and fillers strongly affect the optical, surface and mechanical properties of paper. A very efficient flocculation aid system to deposit fine material (colloids) on fibres (collectors) consists of a polymer used in conjunction with anionic microparticles. The objective of this thesis is to elucidate the mechanism of fines retention with poly(electrolyte)-montmorillonite flocculant systems. Two main subjects are addressed. Firstly, polymer transfer between colloidal particles and collectors was characterised and modelled, and the influence on the papermaking process was evaluated. Secondly, the role of anionic microparticles in a dual flocculation aid was investigated. The influence on the flocculation kinetics and the adsorption behaviour of dissolved and colloidal compounds were studied.The hetero-flocculation of wood fibres and fines induced by polymer and microparticles was analysed in a stirred tank reactor. The deposition of fines on fibres in the presence of polymer is transient. This is attributed to polymer transfer between surfaces upon particle detachment, which leads to the formation of an electro-steric barrier. The resulting polymer layers have limited bridging abilities. A mathematical model describing the simultaneous particle deposition and polymer transfer was developed and validated. The kinetics is determined by three rate constants: a deposition rate constant (k1), a detachment rate constant (k2) and a polymer transfer coefficient (beta). The influence of shear, salt concentration and polymer chemistry on the kinetics was quantified.Microparticles act as bridging agents between adsorbed polymer layers. They provide a stronger bond than single polymer layers. Bond strengthening occurs irrespective of the bridging ability of the adsorbed layers towards cellulosic surfaces. The mechanism is explained in terms of charge densities and polymer layer conformations. The de-layering of montmorillonite is crucial for its efficiency.Finally, the efficiency of montmorillonites as white water scavengers was quantified using model wood colloids and polymers. Dissolved and colloidal substances adsorb on montmorillonites following a low-affinity Langmuir isotherm. Dissolved polymers form molecular monolayers, while colloids adsorb as particles. Montmorillonite was found an efficient scavenger for colloidal material.Montmorillonite used in conjunction with a poly(electrolyte) has three effects in improving fines retention. It augments the bond strength as a bridging agent between adsorbed polymer layers, it allows the retention of colloids coated with inactive polymer layers and finally it scavenges the dissolved and colloidal substances, which would otherwise decrease the poly(electrolyte) efficiency