Microfibrilated cellulose as a model for soft colloid flocculation with polyelectrolytes

Abstract

The effect of polyelectrolyte molecular weight, charge density and dose on the flocculation of Microfibrillated Cellulose (MFC) was investigated. Four types of cationic polyelectrolyte commonly used in industry were chosen: Cationic Polyacrylamide (CPAM), Polyethylenimine (PEI), Polydadmac (PDADMAC) and Poly-aluminium Chloride (PAC). Adsorption isotherm and zeta potential measurements of MFC-polyelectrolyte suspensions quantified polyelectrolyte adsorption behaviour. The flocculation mechanisms, size, strength and type of flocs formed were quantified using static conditions, driven by gravity sedimentation through gel point measurements and dynamic conditions through Focussed Beam Reflectance Measurements (FBRM). Four distinct flocculation behaviours were observed. The first of which was that of a polysalt, PAC. Based on the gel point curve, PAC is unable to create a self-supporting network. FBRM reveals that PAC induces the formation of small and dense flocs, corresponding to an aggregation caused by charge screening. Second, charge neutralisation was observed for PEI with total reflocculation and maximum flocculation occurring at the isoelectric point and corresponding to 85% polymer surface coverage. Third, MFC-PDADMAC flocs presented a total reflocculation and maximum flocculation occurring at 50% coverage and a colloid charge of –10mV or −15mV; this agrees with the formation of polymer patches on MFC. Lastly, low reflocculation and maximum flocculation occurred at half polymer surface coverage for CPAM-MFC flocs, corresponding to a bridging mechanism. Polyelectrolyte maximum surface coverage was found to scale inversely to polymer charge density and was mostly independent of polymer molecular weight and morphology. Finally MFC flocs properties were used as model of soft colloids and analysed for specific industrial applications.