Effect of cationic polyacrylamide on the processing and properties of nanocellulose films

The use of high molecular weight cationic polyacrylamide (CPAM) was investigated to accelerate the drainage of nanocellulose (Microfibrillated Cellulose) suspensions into films. The mechanism was quantified and optimized by measuring the gel point, the lowest solids concentration at which a continuous network is formed. The flocculation of MFC was analysed as a function of the polyelectrolyte dosage, charge density and molecular weight as well as process parameters (drainage time) and material properties. The adsorption isotherms of CPAMs on nanocellulose and their zeta potential curves were also analysed as a function of CPAM charge and dosage. Measured CPAM adsorption capacities for the 50% and 10% charged 13 MDa CPAM onto MFC were 5 mg/g and 8 mg/g, respectively, corresponding to adsorption coverage on cellulose of 0.14 mg/m2 and 0.22 mg/m2. The floc strength and drainability of MFC suspensions were quantified with the gel point as a function of CPAM properties. For all combinations of polyelectrolyte molecular weight and charge density, the gel point of a nanocellulose suspension goes through a minimum with increasing polymer dosage. The minimum gel point was independent of the polyelectrolyte charge density at constant molecular weight. However, it reduced with decreasing CPAM molecular weight, at a constant addition rate. The drainage time of a nanocellulose suspension into a film is reduced by 2/3 by halving the gel point from 0.2 to 0.1 kg/m3; this is due to the more flocculated suspension facilitating drainage between flocs. Nanocellulose films of increased porosity also result from reducing the gel point, signifying that the more open 3D structure of the flocculated cellulose suspension is retained upon drying the 2D film cellulose film structure.

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