Defence of doctoral thesis: Mengxiao Zhao – Study of Paper Dry Strength Additives on Cellulose Fibres
Paper packaging is commonly found in the form of paper and cardboard boxes, but recently paper packing is being used in food packaging, trays, beverage packaging and even paper straws, among others. Paper strength additives have been developed to increase the efficiency of the joint strength and the joint areas and potentially maintain bulk in paper products. In this thesis, a fundamental study of paper dry strength additives was performed involving kraft birch fibres and model cellulose materials to explore the interaction between strength additives and cellulose fibres. The same techniques used in paper strength additives were extended to the adsorption of fire-retardant polymers on fibres.
Adsorption behaviours of cationic starch (CS) and anionic polyacrylamide (APAM) or anionic polyelectrolyte complexes (PECs) were studied on kraft birch fibres with different surface charges. It was found that the saturation adsorption of CS on birch fibres in 0.01 M NaCl showed a linear relationship with the surface charge of fibre, while the adsorption of APAM and PECs was independent of the surface charge of fibres. Higher surface charge of fibres expands the saturation adsorption capacity of strength additives, consequently, improves the tensile strength properties of handsheets made of kraft birch fibres with CS/APAM or CS/PECs. Bending stiffness of handsheets was improved by CS and boosted by PECs or APAM. In addition, sheet density was not significantly compromised by adding strength additives. Cationic strength additives were investigated on model cellulose surfaces, model cellulose nanofibrils filaments, and model cellulose beads. CSs produced an increase in the mass of the cellulose surface, while cationic PAMs led to a decrease in the total mass of cellulose surface determined by Quartz Crystal Microbalance with Dissipation monitoring. All types of strength additives affected the filament joints but in different ways, while most of them made filaments form intermixed interphases in the joints. The adsorption of chitosan and sodium hexamethaphosphate increased steadily and formed three bilayers on cellulose fibres. These materials formed stable lightweight materials after freeze-drying suspensions of the coated fibers containing them and show good fire retardant properties.
The fundamental study of paper dry strength additives and other polymers on the cellulose fibres can facilitate the development of new polymers for wood fibre-based materials.